Top discharge roof ventilator

ABSTRACT

A roof ventilator for exhausting air includes a tapered, upstanding, hollow stack for sealed securement on a roof at an air opening therein. A hollow, upstanding and semispherical outlet bell is telescoped in close spaced radial relation over the upper end of the stack and has an open upper end. A substantially flat damper pan normally rests upon and closes the upper stack end. A mounting sleeve is fixedly supported within the stack. A hollow guide element, pendently and coaxially fixed to the damper pan, is slidably located within the mounting sleeve and carries limiting means coacting with the mounting sleeve for limiting movement of the damper pan upwardly off the upper stack end, whereby a pressure drop from the stack to the bell causes the damper pan to lift from the upper end of the stack to vent the stack into the bell.

United States Patent [1 1 Doherty, II

[ TOP DISCHARGE'ROOF VENTILATOR [75] Inventor: Lewis S. Doherty, H, Parish of East Baton Rouge, La.

[73] Assignee: Doherty-Silentaire, Baton Rouge,

[22] Filed: May 26, 1972 [2]] Appl. No.: 257,148

[56] References Cited UNITED STATES PATENTS 3,401,624 9/1968 Mohrman 1. 98/119 X 1 1,276 7/1854 Thompson 98/42 X 1,656,349 1/1928 Garrison l37/533.21 X

2,084,338 6/1937 l-lamblin 98/64 2,368,320 1/1945 Norris...;.. 98/43 3,012,495 12/1961 Miller 98/116 3,313,228 4/1967 Cook 98/43 R Jan. 29, 1974 3,472,150 10/1969 Strawsine... 98/116 Primary Examiner- William ODea Assistant Examiner-Peter D. Ferguson Attorney, Agent, or Firm-Woodhams, Blanchard &

Flynn [5 7] ABSTRACT A roof ventilator for exhausting air includes a tapered, upstanding, hollow stack for sealed secu'rement on a roof at an air opening therein. A hollow, upstanding and semispherical outlet bell is telescoped in close spaced radial relation over the upper end of the stack and has an open upper end. A substantially flat damper pan normally rests upon and closes the upper stack end. A mounting sleeve is fixedly supported within the stack. A hollow guide element, pendently and coaxially fixed to the damper pan, is slidably located within the mounting sleeve and carries limiting means coacting with the mounting sleeve for limiting movement of the damper pan upwardly off the upper stack end, whereby a pressure drop from the stack to the bell causes the damper pan to lift from the upper end of the stack to vent the stack into the bell.

12 Claims, 3 Drawing Figures 1 TOP DISCHARGE ROOF VENTILATOR FIELD OF THE INVENTION This invention relates to a roof ventilator and more particularly relates to a top discharge roof ventilator.

BACKGROUND OF THE INVENTION Prior roof ventilators have in general been relatively complex, and hence expensive in construction, particularly as to the arrangement of damper members and baffling therefor. Prior roof ventilators have frequently been of relatively high silhouette, which combined in many instances with a generally displeasing appearance, has degraded the appearance of the building on which they are mounted. The construction of such prior roof ventilator devices as in certain instances been such that portions thereof are susceptible to vibration or rattling in use and/or to deflection in moderate or high winds. Further, dampers often used on prior art roof ventilator devices have been relatively heavy, have been supported in a relatively unbalanced way and/or have been provided with friction mounting arrangements, requiring a substantial pressure drop from the building to the atmosphere in order to open, and thereby requiring the use of a closely adjacent, normally structurally coupled, exhaust fan. Frequently, there is a substantial drop in pressure, or air flow loss, across open or partially open prior art damper structures. In many instances prior upwardly opening devices have provided inadequate protection from entrance of rain, hail or the like past the damper unit, particularly where the damper is open or partially open and air flow rates are relatively low. Further, such prior constructions have often been susceptible to opening of damper members by wind, allowing back drafts into the building.

Accordingly, the objects of this invention include provision of:

l. A top opening discharge ventilator device, or eX- haust valve, for discharge of air from the interior of a building structure adapted for mounting on the roof of such structure.

2. A device, as aforesaid, of low silhouette and pleasing appearance shaped to minimize wind resistance, which is structurally stable under high wind or variable wind conditions with little or no flutter, vibration, etc. and has little or no tendency to lift or change position or configuration under high wind conditions.

3. A device, as aforesaid, of simple and lightweight construction which, particularly in its exterior elements, is of substantial strength and stability of configuration and which is adapted for mounting on a roof without need for reinforcement of such roof and which indeed may be installed on an existing roof surface by a simple mopping-in operation.

4. A device, as aforesaid, which is of inexpensive construction and is manufacturable of commonly available materials in large or small quantities, in which major elements are particularly adaptable for production by a spinning operation from sheet aluminum, which is capable of being stored and shipped in a compact disassembled condition with the elements thereof in nested relation and occupying a volume not substantially exceeding the volume not normally occupied by the larger ones of such elements and which can readily be assembled by persons of little skill or experience with manipulation of a minimal number of fastening devices and in relatively little time.

5. A device, as aforesaid, including an openable damper member or valve which is of relatively lightweight, has relatively low opening friction and is capable of opening in response to a relatively small pressure drop from within the supporting building structure to the outside atmosphere, which is openable in response to the pressure created within the building by an adjacent fan or the like or, alternatively, by other remotely located means, including for example air conditioners, window fans, etc. and in which air flow loss caused by the damper or valve along or in inner action with the surrounding exterior element is minimized.

6. A device, as aforesaid, which though upwardly opening prevents entry of rain, hail or the like therethrough into the building whether the damper valve is open or closed, which provides for drainage of water due to rain or the like from the upwardly opening element thereof while minimizing or eliminating downward or sideward direction of exhausting air toward the supporting roof.

7. A device, as aforesaid, in which the damper or valve is not susceptible to opening by ambient winds, for example when exhaust air flow is small or absent.

8. A device, as aforesaid, which eliminates the need for a sealing shutter or valve within the building structure in the air flow path through the device. 9. A device, as aforesaid, in which the pressure drop required to open the damper, and the air flow through the device, is substantially unchanged despite rain or the like entering the open upper end of the device and striking the damper member.

Other objects and purposes of this invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a pictorial view of a roof ventilator device embodying the present invention and installed on the roof of a building.

FIG. 2 is an enlarged central cross-sectional view, in elevation, of the device of FIG. 1 and showing same installed in conjunction with a motor driven fan.

FIG. 3 is a sectional view substantially taken on the line Ill-III of FIG. 2.

SUMMARY OF THE INVENTION The objects and purposes of the invention are met by providing a roof ventilator for exhausting air which includes a tapered upstanding hollow stack for sealed securement to a roof at an air opening therein. A hollow upstanding and semispherical outlet bell is telescoped in close spaced radial relation over the open upper end.

of the stack and has an open upper end. A substantially flat damper pan normally rests upon and closes the upper stack end. A mounting sleeve is fixedly supported within the stack. A hollow guide element, pivotally and coaxially fixed to the damper pan, is slidable within the mounting sleeve and carries limiting means coacting with the mounting sleeve for limiting movement of the damper pan upwardly off the upper stack end, whereby a pressure drop from the stack to the bell causes the damper pan to lift from the upper end of the stack to vent the stack into the bell.

DETAILED DESCRIPTION The ventilator device embodying the invention is adapted to be supported upon a roof structure, generally indicated at 12, of a building.

The device 10 includes a hollow, open ended stack 14. The stack 14 has a peripheral wall 15 which is upwardly tapered and of generally frustoconical form. A radial flange 16 extends outwardly from the lower end of the peripheral wall 15 and is adapted to be supported upon and secured to the roof structure 12. An opening 17 in the roof structure 12 provides communication between the interior of the building covered by the roof structure 12. The opening 17 is preferably coextensive in size and coaxial with the open lower end of the stack 14. The upper end of the peripheral wall 15 is preferably provided with a reinforced edge 18, in the preferred embodiment shown an outwardly rolled lip.

A hollow tubular mounting sleeve 21 is located coaxially within the stack 14 in close spaced relation to the upper end thereof. The lower end of the mounting sleeve 21 is substantially spaced from the lower end of the stack 14. The mounting sleeve 21 is fixed to the stack by a support here comprising a pair of diametrally aligned support tubes 22 and 23 extending radially inwardly from the peripheral wall 15 of the stack to the upper portion of the guide element 21. The tubes 22 and 23 are spaced below the upper end of the stack. The ends of the support tubes 22 and 23 are preferably secured as by welding to the stack peripheral wall 15 and mounting sleeve 21. The radially inner ends of the support tubes 22 and 23 are preferably flattened in a vertical plane, as indicated at 25, to provide an increased length of contact axially of the guide element 21 and thereby a more rigid securement of the support tubes to the guide element.

A valve element or damper 27 has a diameter slightly exceeding that of the upper end of the stack 14. When at rest, the damper 27 covers the upper end of the stack for closing same. The damper 27 may be somewhat upwardly convex, but is preferably flat and panlike. The damper pan 27 has a circumferentially continuous, pendant flange 29 at the circumferential edge thereof. The flange 29 is preferably sloped, substantially corresponding in taper to the stack 14. The flange 29 extends downwardly past the reinforced upper edge 18 of the stack in snugly surrounding but freely removable relation therewith, thereby overlapping the upper portion of the stack. The flange 29 preferably terminates in a rolled reinforcing rim 30.

A preferably tubular guide element 32 coaxially depends from the underside of the damper pan 27 and is preferably flxed thereto by welding. The guide element 32 axially slidably extends through the mounting sleeve 21. The guide element 32 preferably substantially corresponds in length to the height of the stack 14. A stop element 33, here a cotter pin, extends diametrally through the lower portion of the guide element 32 for preventing unintended upward removal of the guide element 32 from the mounting sleeve 21. The tubular elements 21 through 23 and 32 are preferably thin walled tubular aluminum stock and the damper pan 27 is preferably spun from aluminum sheet.

A weather shield member is located above the stack and, in the preferred embodiment shown, comprises an outlet bell 36 which extends upwardly from the stack 15 and is substantially semispherical, that is convexly rounded, in cross-section. The outlet bell 36 is hollow and has open upper and lower ends terminating in substantially axially extending flanges 37 and 38, respectively. The flanges 37 and 38 assist in reinforcing the open ends of the bell. The upper flange is substantially cylindrical. The lower flange 38 surrounds the upper end of the stack in close spaced relation thereto. The lower flange 38 is preferably upwardly tapered to the same extent as the stack 14.

The outlet bell 36 is fixed at its lower flange 38 to the peripheral wall 15 of the stack 14, preferably removably, as by a plurality of circumferentially spaced nut and bolt connections 40. The lower bell flange 38 is preferably spaced from the stack peripheral wall 15 by interposition therebetween of washers, or the like, carried by the bolts 40, to afford a narrow annular drain opening 41 between the opposed surfaces of the bell and stack. Thus, rainwater entering the top of the bell 36 drains from the lower end of the bell along the outside of the peripheral stack wall 15, through the annular drain opening 41.

The lower bell flange 38 is spaced somewhat below the damper flange 29, here by a distance corresponding substantially to the axial length of such damper flange. The spacing between the lower bell flange 38 and stack peripheral wall 15 is preferably substantially less than the spacing between the lower bell flange 38 and damper pan flange 29. The radial spacing of the bell 36 from the damper pan flange 29 preferably substantially corresponds to or exceeds the axial dimension of such pan flange. Thus, with the damper pan 27 raised somewhat from its closed position of FIG. 2, air from the stack tends to flow upwardly into the bell rather than downwardly through the drain opening 41.

in the preferred embodiment shown, the bell 36 is constructed in two parts, a semispherical upper part 43 and a semispherical lower part 44. The parts 43 and 44 are snugly joined, preferably at the maximum diameter 46 of the bell 36 and by means of a radially outwardly offset depending annular flange 47 on the upper part 43 which overlies the lower bell part 44 with a snug friction fit. Provision of the bell in two parts, as above discussed, facilitates formation of the bell by spinning from relatively thin aluminum sheet material and also enables the device 10 to be knocked down into a more compact package for shipping or storage.

The upper bell flange 37 is preferably substantially equal in diameter to the open upper end of the stack 15, and hence is less in diameter than the lower bell flange 38. The maximum diameter of the damper pan 27 including its flange and rim preferably substantially corresponds to the diameter of the lower bell flange 38.

The location of the stop element 33 allows opening, upward movement of the damper pan 27 to a position at or near the maximum diameter level 46 of the bell 36 whereby the annular air flow zone defined between the periphery of the damper pan 27 and the interior side wall of the bell 36 is maximized with the damper pan fully opened. The annular air flow path surrounding the periphery of the damper pan 27 increases in cross section as the damper pan moves upwardly from the upper end of the stack, by reason of the increase in axial distance between the damper pan and the upper stack end and also by reason of the increased radial distance between the damper pan periphery and the radially opposed portion of the peripheral wall of the bell 36.

In many instances it will be desired to use the device for venting a building maintained at a slight pressure above exterior atmospheric pressure, as by input ventilation fans, air conditioning or the like. Particularly in such instances, the stack flange 16 may simply be placed upon the surface of the roof of the building in coaxially surrounding relationship with a vent hole therein, corresponding to the hole 17 shown in FIG. 2. Thereafter, the flange 16 may be secured to the roof as desired and then covered with a conventional roof sealing membrane, not shown, to prevent leakage of water radially inwardly along the lower surface of the flange l6 and into the opening 17.

However, in other instances it may be desired to install the device 10 in direct conjunction with an exhaust fan or the like, such as generally indicated as 50. Particularly in such instances it may be desired to mount the device 10 on a raised roof deck portion, as generally indicated at 51. To this end, the roof 54 of the building is here disclosed as including an enlarged exhaust opening 55 surrounded by an upstanding laterally closed barrier 56, of any conventional nature such as a square or rectangular enclosure of wood, sealed by any convenient means to the roof deck 54. A carrier plate 58 is fixed to and supported upon the barrier 56 by any convenient means and may be provided with an integral, or separate, depending sealing and locating flange 59. The exhaust fan unit 50 may be of any conventional nature and here is illustrated as including an annular mounting rim 61 having a radial portion 62 fixed by any conventional means to the underface of carrier plate 58 and further having a dependent support portion 63. A motor mounting member 65 carries an upstanding conventional electric motor 67, preferably in coaxiality with the device 10. A plurality of radial arms 68 support the motor mounting member 65 on the rim 61. A fan blade 70 is loosely located within the support portion 63 below the device 10 and is secured to the shaft 71 of the motor 67 for rotation therewith in a conventional manner for impelling air from the area below the roof 54 up and out through the device 10.

OPERATION Portions of the device 10 are, when disassembled, readily nested for storage or shipping. In a preferred nesting arrangement, the inverted upper bell portion 43 is substantially fully nested within the lower bell portion 44. The inverted stack is then nested substantially within the inverted upper bell portion 43. The damper pan 27 is nested rightside up within the inverted stack 14, the damper guide element 32 being received in the inverted mounting sleeve 32. The resulting nested assembly does not substantially exceed the height of its longest element.

The device 10 is readily assembled, for example from such a nested condition, by securing the lower bell portion 44 over the narrow upper end of the stack 14 in the position of FIG. 2 via the nut and bolt connections 40. The spacers or washers 39 are disposed between the bell and stack to provide annular drain opening 41. Thereafter, the guide element 32 of the damper pan may be dropped downwardly into the mounting sleeve 21 and the cotter pin 33 may be placed on the guide element 32, the damper pan 27 resting atop the stack 14. Finally, the flange 47 of the upper bell portion 43 is frictionally fitted over the open upper end of the lower bell portion 44 as shown in FIG. 2.

To install the assembled device 10 on a roof or the like, the stack flange 16 is placed upon the surface, here as shown in FIG. 2 the surface 58, upon which it is to be supported, in coaxiality with the exhaust hole 17 therein. The stack flange 16 is then secured by any convenient means, not shown, such as nails, screws or the like, to the supporting surface 58 and a suitable waterproofing membrane may be applied over the flange 16 and the adjacent portion of the supporting surface 58 in sealed relation therewith. If desired, a waterproofing adhesive or the like may be used between the flange l6 and surface 58 to provide additional water sealing and/or securement of the device 10 to the roof. As stated, the device 10 may be installed either on a built-up deck, as shown in FIG. 2, or on the surface of the roof and with or without an adjacent exhaust fan unit 50.

In operation, with the device 10 in its rest condition shown in FIG. 2, and thus with a zero or substantially zero pressure drop from within the stack to within the bell, the damper pan 27 is seated on the upper end of the stack. This closes the upper end of the stack against entry of drafts, rain or the like and makes unnecessary provision of an auxiliary shutter or the like within the building to close the opening into the stack.

Rain entering the open upper end of the bell 36 runs down the interior sidewalls thereof and exits through the narrow annular drain 41 and then runs down along the exterior of the stack 14. Winds of relatively high velocity do not tend to shift or move the damper pan 27 from its closing relationship with the stack, the damper pan being well shielded from such winds by the bell, the closest open portion of the bell being the narrow annular drain slot 41. Though of relatively light gauge material, the tapered configuration of the stack 14 and rounded configuration of the bell 36 substantially strengthens same and, particularly in the case of the bell, reduces resistance to wind flow therearound. Further, the configuration of the stack and bell has substantial mechanical stability and resistance to vibration and hence is substantially free of flexing and noise production even in high steady or pulsing winds.

In view of the light weight and low mounting friction of the damper pan 27, a relatively low pressure drop from the stack 14 to the bell 36, for example in the nature of a fraction of a pound per square inch is sufficient to raise the damper pan 27 from the upper end of the stack. Raising the damper pan 27 opens the upper stack end and provides venting air flow upwardly out of the building through the stack 14, past the pan 27 and out the open upper end of the bell. Despite the downwardly directed flange 29 on the damper pan 27, a great majority of air flow out the upper end of the stack is smoothly directed, even at relatively low damper pan openings and air flow rates, radially outwardly and then smoothly upwardly by the outwardly and upwardly curving peripheral wall of the lower bell portion 44. Little, if any, significant portion of exhaust air flow passes downwardly through the narrow and rather restrictive annular drain slot 41. Air flow continues in a smoothly curved manner upwardly into the upper bell half 43 and out the upper end thereof.

As increasing pressure drops from the interior of the stack to the interior of the bell, the damper pan 27 rises increasingly greater distances from the upper end of the stack, thus providing an increasingly greater crosssection air path between the damper pan and the upper stack end an lower bell peripheral wall. As relative pressure in the stack 14 increases, the damper pan 27 eventually reaches its maximum height, corresponding to the maximum bell diameter, that is, the region 46 of the bell whereat the radial spacing between damper pan and the bell wall (and hence the air flow path cross-section) is maximized.

Even when the damper pan 27 is in a partially or fully opened conditon (which can be achieved in the inventive device 10 with a relatively small pressure drop across the pan and a low air flow rate), rain or the like falling into the open upper end of the bell does not tend to enter the stack. More particularly, the damper pan whether open or closed, radially overlies the open upper end of the stack and thus prevents vertical rain fall from entering the stack. Further, the surrounding side walls of the bell prevents wind driven rain or the like, falling on a slope, from entering the vertical space between the damper pan and the stack.

The upper surface of the damper pan is preferably flat as shown, or if desired may be somewhat upwardly convex, so that rain water falling thereon will tend to drain off therefrom at its peripheral edge. Thus, the relationship of damper pan height to air pressure in the stack is not influenced by a buildup of rain water or the like atop the damper pan. Further, the high heat conductivity of the damper pan tends to prevent any build up of ice or the like thereon, particularly when venting heated air from the building interior. The high heat conductivity of the stack 14 tends to keep the annular drain opening 41 open for removal of snow, sleet or the like in the form of water.

Upon reduction of the air pressure in the stack 14 to a level equal or substantially equal to that in the bell, the damper pan falls, guided by sliding motion of guide element 32 in the mounting sleeve 21, to the top of the stack for closing same again.

Although a particular preferred embodiment of the invention has been disclosed in detail for illustrated purposes, it will be recognized that variations or modifications of the disclosed apparatus including the rearrangement of parts, lie within the scope of the present invention.

I claim:

1. A roof vent device for exhausting air, comprising in combination:

an upstanding hollow stack for mounting on a roof at a vent opening, said stack having open upper and lower ends;

a hollow upstanding outlet bell fixed with respect to said stack, said bell having an open lower end telescoped over the upper end of said stack and an open upper end, the diameter of said bell intermediate the ends thereof substantially exceeding the diameter of the ends thereof;

a damper pan normally closing the upper end of said stack and located within said bell;

means guiding said damper pan for substantially coaxial movement away from the stack in response to a pressure drop from said stack to said bell so as to exhaust air from said roof opening through said stack and bell.

2. The device of claim 1 in which said bell is substantially semispherical, having convexly outwardly curved sidewalls extending radially outwardly from said ends thereof toward the intermediate portion thereof, said stack being of upwardly tapered frustoconical form and extending for a minor portion of its length into the in terior of said bell to a level below said intermediate bell portion, said guiding means including limit means limiting movement of said damper pan away from said stack to a level approximating the maximum diameter of said bell.

3. The device of claim 2 in which the diameter of said damper pan approximates that of the upper end of said stack, said damper pan having a dependent perimetral flange adapted to overhang said upper end of said stack and being disposed in close radially spaced relation to the exterior face of said stack for assisting in centering said damper pan on said stack as said damper pan is closed and preventing entry of foreign material into said stack when said damper pan is closed, said damper pan resting upon the upper end of said stack when closed.

4. The device of claim 3 in which the lower portion of the curved sidewall of the bell is spaced downwardly from and radially outwardly from said flange of said damper pan, said lower portion of said bell peripheral wall extending upwardly and outwardly in curved spaced relation past said flange for deflecting air passing between said damper pan and the upper end of said stack smoothly outwardly and upwardly through said open upper end of said bell.

5. The device of claim 4 in which the open lower end of said bell is fixed in radially outwardly spaced relation to said stack, whereby rain entering the upper bell end exits at the lower bell end along the outer face of said stack, the minimum spacing of said damper pan from the wall of said bell substantially exceeding the minimum spacing of said bell from said stack whereby' to minimize air flow from said stack downwardly along the peripheral wall of said stack out of said bell.

6. The device of claim 1 in which the diameters of said damper pan and the open upper end of said top bell are approximately equal, the diameter of said bell intermediate its ends substantially exceeding the diameter of said damper pan for providing ample cross sectional area for relatively high volume and low velocity air flow in an annular zone between said damper pan and the peripheral wall of said bell.

7. The device of claim 6 in which the upwardly facing surface of said damper plate is substantially flat for preventing accumulation of rain water thereon, whereby said pressure drop is substantially uniform despite varying weather conditions.

8. The device of claim 1 in which said bell comprises a bowl-like lower bell portion and a bowl-like upper bell portion removably securable thereto by means including an overlapping flange, said overlapping flange being located substantially at the maximum diameter of said bell, said bell portions and stack being compactly nestable when disassembled, said stack and bell portions being dimensioned to allow inverted reception of said upper bell portion in said lower bell portion and inverted reception of said stack in said upper bell portion, said pan being nestable therewith.

9. The device of claim 1 wherein said means quiding said damper pan comprises a guide element coaxially fixed to and dependent from said damper pan, a hollow sleeve coaxially located within said stack adjacent the upper end thereof, arms fixed to the peripheral wall of said stack and extending inboard thereof, said arms fixedly carrying said sleeve therebetween, said arms and sleeve being spaced below and adjacent said damper pan when said damper pan rests upon the top of said stack.

10. The device of claim 9 in which the length of said guide element approximates the height of said stack, said guide element having means adjacent the lower end thereof for coacting with the lower end of said sleeve to limit upward movement of said damper pan to a height corresponding to the maximum diameter of said bell.

11. The device of claim 1 in which said stack is upwardly tapered and substantially frustoconical in form, said bell having a dependent flange at-the open lower end thereof, said flange tapering in substantial parallelism to said stack and lying in close spaced relation thereto for defining an annular water drain opening therebetween for said bell, a plurality of fastening devices connecting said flange to said stack for supporting said bell, said fastening devices being distributed around the periphery of said stack, washerlike elements on said fastening devices disposed between said flange and said stack for maintaining said drain opening.

12. A roof vent device for exhausting air, comprising the combination:

an upstanding hollow stack adapted to be secured in sealed relation on a roof for venting an opening therethrough and having open upper and lower ends;

a hollow weather shield fixed with respect to said stack, said weather shield having an open lower end overhanging the upper end of said stack, an open upper end above said stack and a widened intermediate portion;

a damper pan normally closing the upper end of said stack and located within said weather shield;

means guiding said damper pan substantially coaxially upwardly from the stack to a full open position adjacent the widened intermediate portion of said weather shield in response to an increasing pressure drop from said stack to said weather shield, for maximizing exhaust air flow from said roof opening upwardly through said weather shield. 

1. A roof vent device for exhausting air, comprising in combination: an upstanding hollow stack for mounting on a roof at a vent opening, said stack having open upper and lower ends; a hollow upstanding outlet bell fixed with respect to said stack, said bell having an open lower end telescoped over the upper end of said stack and an open upper end, the diameter of said bell intermediate the ends thereof substantially exceeding the diameter of the ends thereof; a damper pan normally closing the upper end of said stack and located within said bell; means guiding said damper pan for substantially coaxial movement away from the stack in response to a pressure drop from said stack to said bell so as to exhaust air from said roof opening through said stack and bell.
 2. The device of claim 1 in which said bell is substantially semispherical, having convexly outwardly curved sidewalls extending radially outwardly from said ends thereof toward the intermediate portion thereof, said stack being of upwardly tapered frustoconical form and extending for a minor portion of its length into the interior of said bell to a level below said intermediate bell portion, said guiding means including limit means limiting movement of said damper pan away from said stack to a level approximating the maximum diameter of said bell.
 3. The device of claim 2 in which the diameter of said damper pan approximates that of the upper end of said stack, said damper pan having a dependent perimetral flange adapted to overhang said upper end of said stack and being disposed in close radially spaced relation to the exterior face of said stack for assisting in centering said damper pan on said stack as said damper pan is closed and preventing entry of foreign material into said stack when said damper pan is closed, said damper pan resting upon the upper end of said stack when closed.
 4. The device of claim 3 in which the lower portion of the curved sidewall of the bell is spaced downwardly from and radially outwardly from said flange of said damper pan, said lower portion of said bell peripheral wall extending upwardly and outwardly in curved spaced relation past said flange for deflecting air passing between said damper pan and the upper end of said stack smoothly outwardly and upwardly through said open upper end of said bell.
 5. The device of claim 4 in which the open lower end of said bell is fixed in radially outwardly spaced relation to said stack, whereby rain entering the upper bell end exits at the lower bell end along the outer face of said stack, the minimum spacing of said damper pan from the wall of said bell substantially exceeding the minimum spacing of said bell from said stack whereby to minimize air flow from said stack downwardly along the peripheral wall of said stack out of said bell.
 6. The device of claim 1 in which the diameters of said damper pan and the open upper end of said top bell are approximately equal, the diameter of said bell intermediate its ends substantially exceeding the diameter of said damper pan for providing ample cross sectional area for relatively high volume and low velocity air flow in an annular zone between said damper pan and the peripheral wall of said bell.
 7. The device of claim 6 in which the upwardly facing surface of said damper plate is substantially flat for preventing accumulation of rain water thereon, whereby said pressure drop is substantially uniform despite varying weather conditions.
 8. The device of claim 1 in which said bell comprises a bowl-like lower bell portion and a bowl-like upper bell portion removably securable thereto by means including an overlApping flange, said overlapping flange being located substantially at the maximum diameter of said bell, said bell portions and stack being compactly nestable when disassembled, said stack and bell portions being dimensioned to allow inverted reception of said upper bell portion in said lower bell portion and inverted reception of said stack in said upper bell portion, said pan being nestable therewith.
 9. The device of claim 1 wherein said means quiding said damper pan comprises a guide element coaxially fixed to and dependent from said damper pan, a hollow sleeve coaxially located within said stack adjacent the upper end thereof, arms fixed to the peripheral wall of said stack and extending inboard thereof, said arms fixedly carrying said sleeve therebetween, said arms and sleeve being spaced below and adjacent said damper pan when said damper pan rests upon the top of said stack.
 10. The device of claim 9 in which the length of said guide element approximates the height of said stack, said guide element having means adjacent the lower end thereof for coacting with the lower end of said sleeve to limit upward movement of said damper pan to a height corresponding to the maximum diameter of said bell.
 11. The device of claim 1 in which said stack is upwardly tapered and substantially frustoconical in form, said bell having a dependent flange at the open lower end thereof, said flange tapering in substantial parallelism to said stack and lying in close spaced relation thereto for defining an annular water drain opening therebetween for said bell, a plurality of fastening devices connecting said flange to said stack for supporting said bell, said fastening devices being distributed around the periphery of said stack, washerlike elements on said fastening devices disposed between said flange and said stack for maintaining said drain opening.
 12. A roof vent device for exhausting air, comprising the combination: an upstanding hollow stack adapted to be secured in sealed relation on a roof for venting an opening therethrough and having open upper and lower ends; a hollow weather shield fixed with respect to said stack, said weather shield having an open lower end overhanging the upper end of said stack, an open upper end above said stack and a widened intermediate portion; a damper pan normally closing the upper end of said stack and located within said weather shield; means guiding said damper pan substantially coaxially upwardly from the stack to a full open position adjacent the widened intermediate portion of said weather shield in response to an increasing pressure drop from said stack to said weather shield, for maximizing exhaust air flow from said roof opening upwardly through said weather shield. 